The present invention relates to a space frame which is used for a long-span roof of a building, or a long-span framing structure used for an artificial foothold to construct a building over a road, rails, a river or other impediment. Especially, the present invention relates to a space frame in which square steel tubular members are used for bar members and joined at each cross point thereof by welding.
A space frame is a well-known device. Especially as shown in FIGS. 1 through 3, a space frame having an upper frame body 2 constructed with upper bar members 1 and 1' joined perpendicularly to form a lattice pattern having square grids; a lower frame body constructed with lower bar members 3 and 3' also joined perpendicularly to form a grid pattern having square grids wherein the upper and lower frame bodies 2 and 4 are arranged in a configuration such that respective bar members of the upper and lower frame bodies make an angle of about 45 degrees with respect to the planes of the frame bodies, and the upper and lower frame bodies 2 and 4 are united by joining a number of sets of four diagonal members 5 in the shape of an upended pyramid between cross points of bar members 1 and 1' and cross points of the lower bar members 3 and 3', is long known (refer to, for example, Japanese Published Examined Patent Ser. No. 38-21585=Japanese Patent Ser. No. 443434).
Also, it is well-known to use square tubular members for bar members of a truss in such a way that two perpendicular sides of each square steel tubular member make an angle of about 45 degrees to the plane of the truss frame body, and the bar members are joined by fillet-welding at each cross point for constructing a truss structure (refer to, for example, Japanese Unexamined Utility Model Application No. 62-96403).
The problems of the conventional space frame will be discussed as follows.
(a) Generally, circular hollow section members are used for bar members of a conventional space frame. In this case, the bar members are joined by welding along a curved surface at the joint section, so that this work is very difficult and takes a long time. Square steel pipes are used to settle this problem. The square pipes are generally used in a configuration such that two perpendicular sides of each of the square steel pipes are respectively aligned in parallel and perpendicular to the plane of the frame bodies. In this case, cross points of bar members are joined by butt-welding along a short weld line. This prevents the lower frame body from having enough reliability in weld strength in view of the fact that the lower body must bear a tension load. Further, it is required to inspect welded sections by ultra-sonic flaw detection, which takes long time.
Consequently, a molded steel pole-joint is required to be used at each cross point of the conventional space frame. The bar members are joined with each other through the pole-joint. However, the pole-joint is very expensive, so that it increases the cost of the space frame due to its usage. Also it increases the weight of the space frame by its weight. Moreover, since a joint by the pole-joint is a so-called pin joint, the bar member and the diagonal member can not have enough buckling strength, that is, the buckling load of the whole space frame is caused to be low. This requires the bar member to have larger cross section. As the result of that, the space frame is expensive and heavy.
(b) The truss structure, described in the Japanese Published Unexamined Utility Model Application No. 62-96403, has the feature that the bar member and the diagonal member are tightly joined at the cross points by fillet-welding. However, that application does not disclose and suggest a technical concept relating to a space frame.